1. Field of the Invention
The present invention relates to a liquid crystal display device and an inspection method for a transparent substrate. The present invention relates, in particular, to a liquid crystal display device of D-STN (Double layered Super-Twisted Nematic) mode.
2. Discussion of Background
Various kinds of liquid crystal display device of TN (Twisted Nematic) mode, STN (Super-Twisted Nematic) mode or the like have widely been used. The liquid crystal display device of TN mode is of such a system that a nematic liquid crystal is sandwiched by two glass substrates wherein the direction of long axis of liquid crystal molecules is twisted 90° between the glass substrates. The device of STN mode is of a system that the twist angle is 180° or more in order to obtain a sharp change in the intensity of transmitting light to an applicable voltage.
When a monochrome display of a light source color and a dark appearance is to be performed in the STN mode, a retardation plate or a compensation cell is used in addition to a displaying liquid crystal cell. The system performing the monochrome display of a light source color and a dark appearance by using the compensation cell is referred to as D-STN mode. FIG. 19 is a diagrammatically cross-sectional view of a conventional liquid crystal display device of D-STN mode in the D-STN mode, a displaying cell 101 and a compensation cell 201 are provided between a first polarizing plate 302 and a second polarizing plate 303, and a backlight 301 is disposed at a rear surface side of the first polarizing plate 302. In the following, the side at which the backlight 301 is disposed is referred to as a rear surface side and the side at which the second polarizing plate 303 is disposed is referred to as a front surface side.
In the displaying cell 101, a liquid crystal layer 105 is sandwiched by a pair of glass substrates 102, 108 having transparent electrodes 103, 107. Alignment films 104, 106 are disposed on opposing surfaces of the transparent electrodes 103, 107.
In the compensation cell 201, a liquid crystal layer 205 is sandwiched by a pair of glass substrates 202, 208 having alignment films 204, 206. The liquid crystal layer 105 of the displaying cell 101 and the liquid crystal layer 205 of the compensation cell 201 have a relation that twist angles are the same and the direction of twisting of long axis of liquid crystal molecules in either cell is opposite to that of the other. Further, birefringence values of the two liquid crystal layers 105, 205 are equal. In addition, directions of alignment of liquid crystal molecules in the displaying cell 101 and the compensation cell 201 at sides of adjacent glass substrates are perpendicular to each other. Namely, the direction of alignment of liquid crystal molecules in the liquid crystal layer 105 of the displaying cell 101 at a side of glass substrate 102 is perpendicular to the direction of alignment of liquid crystal molecules in the liquid crystal layer 205 of the compensation cell 201 at a side of glass substrate 208.
The glass substrate 102 at a front surface side of the displaying cell 101 and the glass substrate 208 at a rear surface side of the compensation cell 201 are so designed as to have the same thickness.
If the distance (the air space) between the glass substrate 102 at a front surface side of the displaying cell 101 and the glass substrate 208 at a rear surface side of the compensation cell 201 is too small, interference fringes (Newton rings) are generated due to the air space. Accordingly, in the conventional technique, the thickness of the air space formed between the glass substrate 102 and the glass substrate 208 was determined to have a specified value. The first polarizing plate 302 is bonded to the glass substrate 108 at a rear surface side of the displaying cell 101, and the second polarizing plate 303 is also bonded to the glass substrate 202 at a front surface side of the compensation cell 201. The direction of polarization axis of the first polarizing plate 302 and the direction of polarization axis of the second polarizing plate 302 are perpendicular to each other. Further, it is preferable that a direction of polarization axis of the first polarizing plate 302 and the direction of alignment of liquid crystal molecules in the liquid crystal layer 105 of the displaying cell 101 at a side of glass substrate 108 (a rear surface side) form an angle of 45°, and the direction of polarization axis of the second polarizing plate 303 and the direction of alignment of liquid crystal molecules in the liquid crystal layer 205 of the compensation cell at a side of glass substrate 202 (a front surface side) form an angle of 45°, in order to obtain a display of good light source color.
When a dark appearance is to be displayed, no voltage is applied to the liquid crystal layer 105 of the displaying cell 101, or a voltage lower than the predetermined threshold voltage is applied. A first path 351 shown in FIG. 19 indicates the path of light when the dark appearance is to be displayed. Light from the backlight 301 presents a linearly polarized light after it has passed through the first polarizing plate 302. Further, after passing through the displaying cell 101, it presents an elliptically polarized light. However, the light is again rendered to be a linearly polarized light by means of the compensation cell 201. As a result, the light is interrupted by the second polarizing plate 303 as shown by the first path 351 whereby the liquid crystal display device presents the dark appearance. When the light source color is to be displayed, the state of alignment of liquid crystal molecules is changed by applying a voltage to the liquid crystal layer 105 of the displaying cell 101.
Even in a case that a retardation plate is disposed in place of the compensation cell 201, the same display as in the D-STN mode can be realized. However, the quality of display in using the retardation plate is apt to suffer influence due to a temperature change. Accordingly, the D-STN mode is often used in, for example, an in-vehicle liquid crystal display device having a wide range of temperature change in environment of use.